Question

Why acceleration is called 'g'

Answer 1

The acceleration is called 'g' when it is under the influence of gravity i.e acceleration due to gravity then, it is called 'g'.

Answer 2

g for Gravity.
g is accerlation due to gravity of an object when it falls freely only under the influence of gravity, i.e., in vaccum. In real life, this value may not be constant. It varies over the planet. But we take g as constant.